1. Field of the Invention
The disclosure relates to short detection, and more particularly, to a short detection circuit, an LED driving chip and an LED device thereof, and a short detecting method which are capable of avoiding high-voltage burnout during short detection.
2. Description of the Prior Art
Due to environmental concerns and technology advancement in recent years, light-emitting diodes (LEDs) have gradually replaced cold cathode fluorescent lamps (CCFLs) to become the main trend of screen backlights of computers and TVs. In a conventional LED device, in order to prevent an LED short event from causing high power consumption within an LED driving chip and even burning out the LED driving chip, a short detection circuit is usually disposed in the LED driving chip for detecting LED shorts, and further controlling an LED driving circuit within the LED driving chip to cut off a driving current when an LED short occurs.
Please refer to FIG. 1, which is a schematic diagram of a conventional LED device 10. The LED device 10 includes an LED string C1 and an LED driving chip 102. The LED driving chip 102 includes an LED driving circuit 104 and a comparator 106, i.e. a short detection circuit, for driving and performing short detection on the LED string C1, respectively. Noticeably, FIG. 1 only illustrates one LED string, one comparator and one driving circuit for simplicity. However, the LED device 10 can practically include a plurality of LED strings connected in parallel, a plurality of comparators, and a plurality of driving circuits. Each of the plurality of LED strings is similar with the LED string C1, and is driven and monitored for shorts by the corresponding driving circuit and comparator of the LED driving chip 102.
In detail, the LED driving circuit 104 provides a driving current to drive the LED string C1 according to a feedback voltage Vfb associated with the LED string C1, so as to maintain the feedback voltage Vfb and a boost voltage Vbst within reasonable ranges (e.g., around 20V-40V). The comparator 106 compares a bottom voltage Vbtm of the LED string C1 with a reference voltage Vref (e.g. 5-8 V) within the LED driving chip 102, and determines a short circuit occurs in the LED string C1 when the bottom voltage Vbtm is greater than the reference voltage Vref, so as to further control the LED driving circuit 104 to cut off the corresponding driving current.
Under this configuration, once a short circuit occurs in one or more LEDs of the LED string C1 to make a zero voltage drop across the one or more LEDs, the bottom voltage Vbtm will rise, becoming greater than the reference voltage Vref. At this moment, the comparator 106 can detect that the bottom voltage Vbtm is too high, and therefore determine that a short circuit occurs in the LED string C1, so as to further control the LED driving circuit 104 to cut off the driving current to the LED string C1.
After the LED driving circuit 104 cuts off the driving current of the LED string C1, the bottom voltage Vbtm rises further to a higher voltage level which is almost the same as that of the boost voltage Vbst. For the conventional LED driving chip 102 which is manufactured in a high operating voltage process, it can receive the high voltage level without being damaged. However, in a current trend toward system on chip (SOC) architectures, an LED driving chip is integrated with image processing circuits, and for purpose of achieving higher operating speed, manufactured in a low voltage process to have a low operating voltage often no more than 5V. In consideration of tendency of such a low-voltage single chip to be burned out when receiving a high voltage, there is a need for improvement of the conventional short detection circuit, so as to adapt to the low-voltage single-chip trend.